This study aims at estimating the anchovy biomass that was missed in the echosounder surface blind zone during the PELGAS survey, in an attempt to explain the relative difference between acoustic and DEPM biomass estimates in springtime 2019 in the Bay of Biscay. Acoustic data collected by a side-looking echosounder are analysed in order to estimate the negative bias introduced in acoustic biomass estimates by the acoustic blind zone. Corrected anchovy acoustic biomass estimates and maps are compared to DEPM ones, in an attempt to reconcile egg and acoustic estimates. Remaining discrepancies are discussed in the light of acoustic and egg biomass estimation assumptions.

We showed that surface school biomass can be calculated while applying standard acoustic data analysis methodology to horizontal echosounder data, combined to broadband in-situ TS measurements. In spring 2019, the surface blind zone observation bias was not significant, as only 5% of the anchovy biomass was located in the 0-10m surface layer. The difference between acoustic and DEPM global biomass estimates observed in 2019 hence remains unexplained. Differences between acoustic biomass and egg counts maps were likely due to local differences in fecundity, probably caused, at least in the North Western area, by the presence of shallow schooling large anchovy displaying higher fecundity. This pilot study might pave the way to the routine combination of vertical and horizontal acoustic data to correct surface blind zone bias and improve small pelagic fish acoustic biomass estimates.

DEPM and acoustic biomass estimates have to be calculated within post-stratification regions as homogeneous as possible, to reduce bias and improve estimate precision. The high spatial resolution of acoustic backscatter generally allows to delineate reasonibly homogeneous post-stratification regions. The coarser spatial resolution of the trawl hauls providing biological data used to calculate daily fecundity estimates in the DEPM method so far prevented to assess the spatial heterogeneity of fish fecundity.

Daily Fecundity proxy maps derived from surveys such as PELGAS, where both egg and acoustic data are collected, might be used to assess the spatial heterogeneity of fish fecundity. Those new information could contribute to improve the precision of DEPM estimates, by improving post-stratification region delineation. Assessing the spatial and statistical distributions of daily fish fecundity might hence contribute to improve the precision of mean DF values used in DEPM, and ultimately better explain the discrepancies observed during some years between egg and acoustic indices.